This invention relates to the global positioning system (GPS), GPS receivers, and interference reduction in GPS receiver systems. The invention may be used in rotating weapons, handheld GPS receivers, and any other GPS system requiring a low-cost interference reduction system.
Global positioning systems, such as the US NAVSTAR GPS and Russian GLONASS, are known. The NAVSTAR GPS developed by the U.S. Department of Defense is a satellite-based radio navigation system that transmits information from which extremely accurate navigational calculations can be made in three-dimensional space anywhere on or near the Earth. Three-dimensional velocity can be determined with similar precision. GPS uses eighteen to twenty-four satellites that may be evenly dispersed in three inclined twelve-hour circular orbits chosen to ensure continuous twenty-four hour coverage worldwide. Each satellite uses extremely accurate cesium and rubidium vapor atomic clocks for generating a time base. Each satellite is provided with clock correction and orbit information by Earth-based monitoring stations. Each satellite transmits a pair of L-band signals. The pair of signals includes an L1 signal at a frequency of 1575.42 MHz and an L2 signal at a frequency of 1227.6 MHz. The L1 and L2 signals are biphase signals modulated by pseudo-random noise (PRN) codes and an information signal (i.e., navigation data) encoded at 50 Hz. The PRN codes facilitate multiple access through the use of a different PRN code by each satellite.
Upon detecting and synchronizing with a PRN code, a receiver decodes the PRN encoded signal to recover the navigation data, including ephemeris data. The ephemeris data is used in conjunction with a set of Keplerian equations to precisely determine the location of each satellite. The receiver measures a phase difference (i.e., time of arrival) of signals from at least four satellites. The time differences are then used to solve a matrix of four equations. The result is a precise determination of the location of the receiver in three-dimensional space. Velocity of the receiver may be determined by a precise measurement of the L1 and L2 frequencies. The measured frequencies are used to determine Doppler frequency shifts caused by differences in velocity. The measured differences are used to solve another set of equations to determine the velocity based upon the Doppler phase shift of the received signal.
GPS signals are very low in amplitude and are transmitted using a spread-spectrum signal bandwidth centered at 1575.42 MHz. The GPS signals cover a frequency spread of about 20 MHz. GPS receivers are subject to disruption by jammer signals, which may be transmitted either as narrow band signals or broadband signals. Known GPS receiver systems may reduce the effects of narrow band jamming by using frequency-selective filters, such as notch filters, to attenuate the jamming signal. However, broad band jamming signals are more difficult to reduce or eliminate as the frequency spread of the jamming signals approximates the frequency spread of the GPS signal. Because the frequency spreading sequence of the GPS signal is encrypted according to a pseudo-random noise code, the jammer cannot be precisely synchronized to the GPS signal. This permits the effects of the jamming signal to be reduced by nulling-out the jamming signal. Further, the signal strength of the jamming signal is typically much greater than the signal strength of the GPS signal.
Interference and jamming suppression and/or cancellation are a pressing need in military receivers. Cost and size of interference canceling systems such as null steering or beam forming systems have limited their use to GPS systems installed on the most high value platforms such as aircraft and ships.
Null steering techniques are known in the art. These nulling techniques are based on determining an angle of arrival of a signal based on the phase shift of the signal observed at different antenna elements of an antenna array. Various weights or weight values are assigned to each antenna element and are used to adjust the phase and level of attenuation of the received signal in an attempt to null-out the jammer signal. Power minimization is a known technique that attempts to adjust the weights so as to reduce the total measured power coming from the antenna array. Power minimization techniques rely on the assumption that the jammer signal is much stronger than the GPS signal and that it emanates from a different single direction of arrival. An array having multiple antenna elements is spatially selective so that a null can be placed in the direction of the jammer by adjusting the weight values. Various known algorithms, such as least mean squares and direct matrix inversion may be used to implement the power minimization technique in digital systems. In power minimization techniques, it is assumed that almost all of the signal power is due to the jammer component, and not due to the GPS signal, because the GPS signal is very weak and therefore provides no significant power contribution.
Recent size reductions in GPS receivers have permitted a new capability for them to be use small form factor weapon systems such as bombs, missiles, rockets, and artillery shells. These new platforms are extremely sensitive to cost and sometimes have requirements for an airframe that is continuously rolling and have high launch shock such as artillery shells. Additionally, if they are to be used for military purposes, resistance to interference and jamming is highly desired.
Handheld GPS receivers are widely used in military applications. Handheld GPS receivers would also benefit from a low-cost small size interference reduction system.
What is needed is a very low-cost interference reduction system specifically designed for use in weapons systems having such requirements as small size, a rolling airframe, and high-g acceleration when a shell is fired. It is also desirable to be able to incorporate the low-cost interference reduction system in handheld GPS receivers.
A low-cost interference reduction system for a global positioning system receiver is disclosed. The low-cost interference reduction system may be used in rotating weapons such as artillery projectiles and in hand-held GPS receivers. The low-cost interference reduction system comprises a reference antenna and a secondary antenna for receiving global positioning system signals and interfering signals. A complex weight circuit adjusts the phase and gain of the secondary antenna signal by varying the gain of the in-phase and quadrature component of the secondary antenna signal. A summing function adds the reference antenna signal to the complex multiplied secondary antenna signal to reduce the interfering signals. The GPS receiver processes the global positioning system signals into position solutions and provides a power detector output and a clock output. A microprocessor receives and measures the power detector output and uses the measurement along with the clock output from the GPS receiver to generate the in-phase and quadrature gain signals for the complex weight circuit such that the interfering signals are reduced. Amplifier and filter functions are connected to the reference and secondary antennas, the summing function, and the complex weight circuit to filter and amplify the received GPS and jamming signals. A digital attenuator is connected to the output of the summing function and the microprocessor to reduce the level of the received signals so as to maintain them within the range of an AGC circuit in the GPS receiver. A digital-to-analog converter is connected to the microprocessor and the complex weight circuit to convert the in-phase and quadrature gain signal from digital to analog. The GPS receiver further comprises an AGC function for controlling signal levels in the GPS receiver and for providing the power detector output.
It is an object of the present invention to provide a low-cost interference reduction system for use with GPS receivers.
It is an object of the present invention to provide a low-cost interference reduction system for use in rotating weapons incorporating GPS receivers.
It is an object of the present invention to provide a low-cost interference reduction system for use in hand-held GPS receivers.
It is an advantage of the present invention to maintain interference cancellation in the presence of a high roll rate by the use spin symmetrical antennas in the interference reduction system.
It is an advantage of the present invention to reduce cost by the use of power detector and clock signals from the GPS receiver.
It is a feature of the present invention to reduce interference and jamming signals from several directions.
It is a feature of the present invention to acquire and receive GPS signals and to direct a null toward a jammer when installed in a continually spinning platform.